This invention relates to a process for preparing cyclopentadiene and substituted cyclopentadienes, such as tetramethylcyclopentadiene.
Cyclopentadiene is useful as a monomer and comonomer in the manufacture of polycyclopentadiene and C.sub.5 hydrocarbon resins which are useful in printing inks and adhesives. Substituted cyclopentadienes, particularly tetramethylcyclopentadiene, are useful as ligands in the preparation of homogeneous single site catalysts, which are used in the production of polyolefins, notably linear low density polyethylene. For example, European Patent application publication number 0,277,003 discloses the use of bis(hydrocarbyl-substituted cyclopentadienyl) complexes of zirconium in preparing catalysts for the polymerization of ethylene.
Cyclopentadiene and methylcyclopentadiene can be obtained in small quantities from the C.sub.5 fractions of naphtha crackers, or alternatively, from pyrolysis gasolines. Other substituted cyclopentadienes are more difficult to obtain. For example, the synthesis of tetramethylcyclopentadiene typically begins with the condensation of one mole of diethyl ketone with two moles of acetaldehyde to yield 2,3,5,6-tetrahydro-2,3,5,6-tetramethyl-.gamma.-pyrone, which can be converted under acid treatment to 2,3,4,5-tetramethylcyclopent-2-enone. See for example, InorganicSyntheses, 29, 193, or R. B. King and J. J. Eisch, OrganometallicSyntheses, 3 (Elsevier, 1986), 489. The ketone can be reduced with a reducing agent, such as lithium aluminum hydride, to the corresponding 2,3,4,5-tetramethylcyclopent-2-enol. Dehydration of the tetramethyl-substituted cyclopentenol with aqueous sulfuric or hydrochloric acid yields tetramethylcyclopentadiene. Disadvantageously, the dehydration produces as by-products bis(tetramethylcyclopentenyl) ethers and polymeric cyclopentadienes. In addition, the acid must be reclaimed, or alternatively, must be neutralized creating a waste salt stream.
In view of the above, it would be advantageous to have an efficient method of dehydrating substituted cyclopentenols to substituted cyclopentadienes in high yield. It would be more advantageous if the process did not employ a liquid acid which must be reclaimed or neutralized.